1. Field of the Invention
The present invention relates to a handheld electronic device having a thin housing body. More particularly, the present invention relates to a handheld electronic device, such as a handheld (or laptop) computer or a mobile communication terminal, having a stress releasing structure and to a method for releasing stress imposed on such electronic devices.
As used herein, the term "handheld" is intended to be synonymous with "laptop." Thus, the term "handheld device" would include a "laptop device," and the term "handheld computer" would encompass a "laptop computer," for example.
2. Discussion of the Background
Recently, handheld electronic equipment, such as handheld computers and mobile communication terminals, has been developed having multiple functions and a compact size. Further, keyboard panels have been made with a very thin structure in order to enhance the compact sizing of handheld electronic equipment.
Generally, such handheld electronic equipment is includes a flat housing body, a keyboard device provided in a keyboard mounting opening in the housing body, and a circuit board provided in the housing body. The circuit board faces the keyboard device.
The keyboard device for a handheld computer is constructed by providing a plurality of keycaps and a pointing device on the front face of a keyboard panel. Usually, the bottom surface of the keyboard panel is covered with a metal shielding plate for reducing or preventing switching noises from the key devices. When the keys are depressed or when the pointing device is pushed, the stress imposed upon the keycaps or pointing device is received by the combined structure of the keyboard panel and the covered shield plate.
The circuit board is installed in the housing body with mounting circuit elements, such as a semiconductor package. The mounting circuit elements are very densely arranged.
Generally, it is advantageous for compact handheld electronic equipment to utilize a ball grid array (BGA) type semiconductor package. There are other types of semiconductor packages, such as a quad flat package (QFP). However, it is problematic to utilize a QFP in thin and small-sized equipment because the package size of a QFP increases as the number of input-output pins increases.
On the contrary, since the connecting terminals for the BGA semiconductor package are constructed by a plurality of solder balls that are positioned in a matrix structure on the backside surface of the package body, it is possible to increase the number of the input-output pins without increasing the size of the package. Consequently, the BGA semiconductor package is usually used for thin and small-sized handheld electronic devices.
Recently, there has been demand for thinner housing bodies for handheld electronic equipment. As thinner housing bodies have been developed, the mounting space between the keyboard panel and the circuit board in slim handheld computers has narrowed. For example, the gap between the BGA semiconductor package provided on the circuit board and the keyboard panel in a B5 sized handheld computer is typically less than 1 millimeter (mm). At the same time, the circuit element of the BGA semiconductor package is provided closer to the keyboard panel. Further, the keyboard panel itself is formed by a very thin plate so that the housing body can be made thin. Consequently, the keyboard panel as well as the housing body is becoming weaker and less rigid.
As the number of input-output pins increases in a BGA semiconductor package, the diameter of the solder ball for the BGA semiconductor package becomes smaller so that many pins can be densely arranged. For example, in a handheld computer having a thin housing body, the diameter of the solder ball is only several millimeters. Consequently, the contact surface between the solder ball and the circuit board becomes very small, and the binding force between them also becomes very weak.
In this situation, when a stress is imposed to the keycaps during an input operation to the handheld computer, the keyboard panel elastically deflects (i.e., bends) toward the circuit board because of the low rigidity of the keyboard panel. The bending of the keyboard panel may cause the keyboard panel to contact the BGA semiconductor package mounted on the circuit board. Thus, the stress imposed to the keycaps is directly transferred to the semiconductor package without any attenuation. This may cause cracks to develop at a soldering portion of the solder ball. Consequently, the reliability of the electric connections for the BGA semiconductor package is reduced because of the possibility of disconnected or severed wires caused by the cracks.
Many handheld computers use a pointing device for inputting coordinate positioning information. The pointing device includes a pointing stick and a plurality of distortion sensors provided around the pointing stick on a board. The pointing stick is placed among the keycaps and passes through the keyboard panel. A cylindrical elastic cap is attached to the top of the pointing stick. When the pointing stick is pressed and stressed (e.g., stressed perpendicular to its axis direction), a distortion is detected by one or more of the sensors provided around the pointing stick on the board. As a result of the detection, the inputted directional information moves a pointer on the screen in a desired direction and/or to a desired position.
In a handheld computer having a thin housing body, the use of the pointing device may cause the keyboard panel to bend or deflect downward under the stress imposed to the pointing stick if the keyboard panel is not rigid enough to resist the force applied to the pointing device, as with the operation of the keys. The deflection of the keyboard panel is detected by the sensors, causing the sensors make false detections. Consequently, the inputted directional information does not coincide with the movement of the pointer on the screen. In addition to making the input of directional information more difficult, a user may believe that the pointing stick is malfunctioning.